The present invention relates to a method for compensating for atmospheric turbulence to improve accuracy in a system for free space coherent optical communications and relates to an equalization technique for a femtosecond laser optical combs.
In the case of free space optical communications, if an atmosphere refractive index on an optical path length is changed when laser light is propagated, a phase that the laser light experiences is changed, and the phase changed over time may be observed as a change of a frequency. That is,
  f  =                    d        ⁢                                  ⁢        ϕ            dt        ⁢                  ⁢          (                        f          ⁢                      :                    ⁢                                          ⁢          optical          ⁢                                          ⁢          frequency                ,                  ϕ          ⁢                      :                    ⁢                                          ⁢          optical          ⁢                                          ⁢          phase                    )      The atmosphere reflective index is changed by a change of pressure, a change of temperature, a change of humidity, a change of wind and atmosphere composition, and the like, and continues to be changed over time in an actual outside atmosphere environment. Accordingly, the optical path length of the laser light transmitted by a transmitter in the system for free space coherent optical communications is changed, and finally, a phase of light received by a receiver is changed. Such a phase variation continues to be changed over time, and it is represented by a shift and fluctuation of an optical frequency. Further, a mechanical motion of the transmitter and the receiver may cause the Doppler shift in the optical frequency, and this Doppler shift is finally represented by a shift of the optical frequency received by the receiver. Since the phase change of the received light due to such atmospheric turbulence and Doppler shift increases bit error and causes inter symbol interference (ISI) of transmitted symbols in a process of demodulating signals after receiving data in the optical communications, a technology of accurately measuring an influence due to the atmospheric turbulence and Doppler shift and automatically compensating the measured influence is required to implement free space coherent optical communications.
In order to compensate for the phase change of the optical frequency caused due to the atmospheric turbulence, a carrier recovery technology of locking a phase and a frequency of a local oscillator used to demodulate the data to the transmitted light including the data to compensate for the influence of the atmospheric turbulence was conventionally used, but since such a method requires the same number of local oscillators and phase locked loops for each of optical frequencies used as an optical source for optical communications, it is difficult to apply to a system using several optical frequencies as frequencies for data transmission such as a wavelength division multiplexing (WDM) system because the system becomes complex and is expensive.
Therefore, in the system for free space coherent optical communications, a method of compensating for the change of the phase and frequency of the light caused by the atmospheric turbulence and alleviating complexity is required, and particularly, it is required that the complexity is not increased even in the system using the several optical frequencies as the frequencies for data transmission such as the wavelength division multiplexing (WDM) system.